A typical slide-type hydraulic valve comprises a valve body and a spool, which is accommodated and movable axially in a cylindrical spool-receiving room (also referred to as “bore”) provided in the valve body. The valve body is provided additionally with a plurality of oil grooves, which are formed perpendicularly to the central axis of the spool-receiving room, and with oil passages, which are provided extending from these oil grooves, to allow passage of hydraulic fluid or oil through the valve. Furthermore, the spool comprises lands, which function as seals against flow of oil, and passages, which function as passageways for flow of oil. In this construction of the hydraulic valve, the spool is shifted axially in the spool-receiving room to change the positions of the lands and passages of the spool with respect to the oil grooves of the valve body, so that an intended result can be achieved, for example, a change in the pressure or a change in the rate of oil flow through an oil passage. For shifting the position of the spool, the spool is connected to a lever, which can be manually operated, or it is equipped with an appropriate device so that the hydraulic valve is operated hydraulically or electromagnetically in consideration of the condition where the hydraulic valve is to be applied.
FIG. 9 shows a regulator valve 100 as an example of such hydraulic valve. The valve body 110 of the regulator valve 100 includes a cylindrical spool-receiving room 111. The central part of the spool-receiving room 111 is connected to a first oil groove 121, which lead to a main oil passage (not shown), where oil pressurized by a hydraulic pump (not shown) and adjusted by the regulator valve 100 is delivered. On the left side of the first oil groove 121, a second oil groove 122 is provided to connect to a lubrication oil passage (not shown). In this arrangement; although the spool 130 is biased leftward by a spring 132 provided on the right side of the valve, if the pressure of the main oil passage is fed back through a third oil groove 123, which is provided on the left side of the valve, the spool 130 can shift rightward overcoming the biasing force of the spring 132. Moreover, a fourth oil groove 124 is provided on the right side of the spool-receiving room 111. When a control pressure is supplied into the fourth oil groove 124, this pressure generates a leftward biasing force on the spool 130 additionally to that of the spring 132. In addition, a fifth oil groove 125 provided on the left side of the third oil groove 123 is connected to a drain oil passage.
A land 131, which is provided at the central portion of the spool 130, is positioned in the first oil groove 121. When the leftward biasing force by the spring 132, the rightward biasing force generated by the pressure supplied in the third oil groove 123, and the rightward biasing force generated by the pressure supplied in the fourth oil groove 124 to set the regulator valve pressure, all these forces acting on the spool 130, are in equilibrium, the first oil groove 121 is in fluid communication with the second oil groove 122. In this condition, part of the oil being discharged from the hydraulic pump is led to the lubrication oil passage to maintain the pressure of the main oil passage at a constant pressure (line pressure).
By the way, the valve body 110 of the regulator valve 100 is an article of cast metal produced by die casting, so each of the first oil groove 121, second oil groove 122, third oil groove 123 and fourth oil groove 124 has a draft or slight taper, which is used for facilitating the removal of the die assembly during the production. Because of the presence of a draft, the length of each oil groove in the direction of the axis of the spool 130 is smaller for the part of the oil groove located deeper in the valve body (part located lower in the drawing of FIG. 9) and larger for the part located shallower. Therefore, the force acting around the spool 130 (for example, the force acting on the peripheral surface of the land 131 in a direction perpendicular to the axis of the spool 130) is stronger when the hydraulic pressure is received at a position shallower in each of the oil grooves. As a result, an unbalanced load is generated in a direction from the shallower part to the deeper part of the valve body over the peripheral surface of the spool 130, and this unbalanced load not only disturbs the smooth movement of the spool 130 (hydraulic lock) but also erodes the valve body 110. Furthermore, the unbalanced load can cause a misalignment of the spool 130 in the spool-receiving room 111, and this misalignment, in turn, increases the amount of oil leak.
For alleviating the adverse effects of the unbalanced load acting on the spool, one method is to provide a labyrinth groove on the peripheral surface of the spool. However, forming such a labyrinth groove requires a number of man-hours, and this method no way eliminates the unbalanced load itself. Therefore, the effectiveness of this method is limited. Another method is to grind the inner surfaces of the oil grooves to remove the drafts or slight tapers, which are created during the molding of the oil grooves. However, this method also requires a number of man-hours and increases the production cost substantially.
On this background, it is an object of the present invention to provide a hydraulic valve that maintains the smooth movement of the spool with little erosion of the valve body. This hydraulic valve should eliminate possibility of an unbalanced load to act on the spool, with a low cost, not requiring provision of a labyrinth groove or grinding of the inner surfaces of oil grooves.